1. Field of the Invention
The present invention relates to an electric motor control apparatus that controls an electric motor using a position sensor and to an electric supercharging apparatus using the electric motor control apparatus.
2. Description of the Related Art
In an electric motor control apparatus that controls an electric motor using a position sensor detecting a position of a magnetic pole of a motor rotor, a control characteristic is deteriorated when displacement occurs between a phase according to the position sensor and an actual position of the magnetic pole due to physical positional displacement (attachment error) occurring when the position sensor is attached. Such deterioration may possibly give rise to a problem that, for example, desired torque and efficiency cannot be obtained. Also, in an electric supercharging apparatus that supercharges an internal combustion engine using an electric motor and an electric motor control apparatus, deterioration of a control characteristic of the electric motor control apparatus causes deterioration of a control characteristic of the electric supercharging apparatus and may possibly give rise to a problem that, for example, desired supercharging pressure and efficiency cannot be obtained. It is possible to reduce deterioration of the control characteristic by reducing an attachment error of the position sensor. However, when a reduction of an attachment error is pursued, costs of the electric motor control apparatus and the electric supercharging apparatus are increased.
In order to correct a phase of the position sensor, JP-A-2009-248749 (Patent Document 1) discloses a technique exploiting a fact that an induced voltage can be detected when energization is stopped. According to the technique disclosed in Patent Document 1, because an induced voltage is proportional to a rotation speed of the electric motor, an amplitude of the induced voltage becomes smaller in a low-speed rotation region and accuracy of detection is reduced. However, because the induced voltage is detected only when the electric motor is rotating at or above a predetermined rotation speed, it becomes possible to suppress influences of a reduction in detection accuracy of the induced voltage. The electric motor in the electric supercharging apparatus is energized and driven only when necessary. Hence, it is not necessary to particularly set a period during which energization is stopped to detect the induced voltage. It thus becomes possible to correct a phase of the position sensor within a range of normal operation of the electric supercharging apparatus.    Patent Document 1: JP-A-2009-248749
According to the technique disclosed in Patent Document 1, however, a lower limit value up to which the induced voltage is detectable is set to a rotation speed of the electric motor but an upper limit value is not set. Hence, by stopping energization of the electric motor instantly while the electric motor is rotating at a high speed, considerable torque fluctuation and vibrations occur and there is a problem that the electric motor may possibly be damaged.
Also, in a case where a torque fluctuation, vibrations, and a rotation speed variance occur in the electric motor while the induced voltage is detected, a waveform of the induced voltage is disturbed. Hence, there is another problem that the induced voltage cannot be detected precisely.
Further, displacement occurring between a phase according to the position sensor and an actual position of the magnetic pole is chiefly caused by physical positional displacement occurring when the position sensor is attached. This displacement therefore should fall within a range of an attachment accuracy of the position sensor. Accordingly, in a case where displacement of a phase found by detecting the induced voltage is out of the range, it is advisable to determine that a detection of the induced voltage itself is not performed properly. According to the technique disclosed in Patent Document 1, however, because a range of an amount of phase correction is not set, there is a problem that an inappropriate amount of phase detection obtained when a detection of the induction voltage is not properly performed is set directly.
In addition, displacement occurring between a phase according to the position sensor and an actual position of the magnetic pole includes a temporal delay of a position sensor signal besides physical positional displacement occurring when the position sensor is attached. Given that the temporal delay of the position sensor signal takes a constant value, displacement of a phase is proportional to a rotation speed of the electric motor. Hence, when a temporal delay of the position sensor signal is not negligible, there is a case where a desired control characteristic cannot be obtained by merely setting a certain amount of phase correction.